Audio, video and broadcasting services using ultrahigh frequency (UHF) band were launched subsequent to digital television (DTV). Examples of the audio, video and broadcasting services using UHF band include Terrestrial Digital Multimedia Broadcasting (T-DMB), Digital Video Broadcast-Handheld (DVB-H), Satellite Digital Multimedia Broadcasting, and Digital Audio Broadcasting (DAB). A wavelength of an available frequency band for these services is longer than a length of a mobile phone. For example, when a λ/4 monopole antenna is used, an antenna length is much larger than that of a mobile phone. For example, in case of a T-DMB, an antennal length is about 40 cm. Thus, such an antenna is inconvenient in use and it is difficult to install the antenna inside a mobile phone.
To solve this problem, many attempts have been made to develop small-sized antennas implemented with internal antennas or stubby antennas.
For miniaturization of internal antennas such as a planar inverted F antenna, a microstrip patch antenna, or a dielectric antenna, their electrical length is reduced by using a dielectric material or changing a shape of an antenna element. However, since the internal antennas are mounted only in printed circuit board (PCB) circuits, it is difficult to maintain an omni-directional radiation pattern of a vertically polarized wave due to the PCB circuit vertically mounted in a mobile phone.
Further, an antenna miniaturization technique that adds a gap capacitor to a conventional loop antenna has a problem in that it cannot maintain high antenna efficiency because it does not use a resonance characteristic of an antenna in itself.
A monopole antenna is an antenna that resonates with a length of λ/4, not λ/2, due to an image effect of an antenna ground plane. Examples of the monopole antenna include a whip antenna, a helical antenna, a sleeve antenna, and an N-type antenna. Most of them are external type antennas and have a length of λ/4.
FIG. 1 illustrates a structure of a conventional monopole antenna.
Referring to FIG. 1, the conventional monopole antenna includes an antenna wire 101, a feeder cable 103, and a ground plane 105.
A length from the ground plane 105 to an end of the antenna wire 101 is L. The feeder cable 103 feeds electric power to the antenna wire 101.
A specific frequency at which the length L of the antenna wire 101 is equal to λ/4 is a resonant frequency.
A large current is generated within the antenna wire 101 at the resonant frequency, and the current induces an electric field and a magnetic field, thus making the antenna wire 101 serve as an antenna.
However, as the resonant frequency decreases, the length of the antenna wire must increase.
FIG. 2 illustrates a structure of a conventional helical monopole antenna embedded in a dielectric substance.
Referring to FIG. 2, the conventional helical monopole antenna includes an antenna wire 201, a feeder cable 203, a ground plane 205, and a dielectric substance 207.
The antenna wire 201 is embedded in the dielectric substance 207 and has a predetermined length from the ground plane 205. The feeder cable 203 feeds electric power to the antenna wire 201.
Like in FIG. 1, a specific frequency at which the length of the antenna wire 201 is equal to λ/4 is a resonant frequency. A large current is generated within the antenna wire 201 at the resonant frequency, and the current induces an electric field and a magnetic field, thus making the antenna wire 201 serve as an antenna.
However, as the resonant frequency decreases, the length of the antenna wire 201 must increase.
Therefore, there is a need for a small-sized monopole antenna with length less than λ/4, which can generate a plurality of resonant frequencies and maintain an antenna length constantly.
To reduce the size of the monopole antennas described above, a new technique was proposed which adds an inductance element such as a helical antenna to a disk monopole antenna. This technique can maintain a broadband characteristic, but an installation of an antenna is complicated. Further, since the width and height of the antenna are large, the antenna is difficult to embed in the mobile phone.
Accordingly, there is a need for small-sized antennas that can maintain a broadband characteristic and can be embedded in a mobile phone.
Since the small-sized antennas occupy a small area in a physical view, its bandwidth is limited to maintain good antenna efficiency. The antenna efficiency represents a power ratio of a power radiated from the antenna to a power supplied to the antenna.
Therefore, it is difficult to apply the small-sized antennas to phones or terminals, which provide services using various frequency bands, for example, T-DMB phones, DVB-H phones, UHF communication terminals, T-DMB/cellular hybrid phones, T-DMB/PCS hybrid phones, and DVB-H/GSM hybrid phones.
There is a need for small-sized antennas that can generate a plurality of resonant frequencies and thus provide multiple resonances, that is, a wideband transmission and reception.
As described above, there is a need for small-sized antennas that have a reduced size, maintain omni-directionality in a mobile phone or the like, is easily installed, have high antenna efficiency, and provide a wideband characteristic.